1. Field of the Invention
The present invention relates to an optical scanner, and more particularly to an optical scanner in which a mirror substrate supported by a pair of torsion springs is oscillated thereby as a turning rotational axis.
2. Discussion of the Background
Optical scanners deflecting and scanning light beam such as a laser beam with a mirror member are widely used for optical instruments such as electrophotographic copiers, laser beam printers and barcode readers. In addition, they are used in displayers scanning a laser beam and projecting an image as well. Particularly, the following optical scanners are disclosed as compact optical scanners.
An optical scanner using a piezoelectric element to drive a mirror member is known. Japanese published unexamined application No. 2005-128147 discloses an optical scanner in which two pairs of piezoelectric unimorph oscillation plates piezoelectric elements are formed on is connected to a mirror member and an antiphase AC voltage is applied to the piezoelectric elements of the two pairs of piezoelectric unimorph oscillation plates to oscillate the mirror member back and forth around a torsion spring as a rotational axis.
An optical scanner driving a mirror member with an electromagnetic force is known as well. For example, Japanese Patent No. 3584595 discloses an optical scanner (light deflection element) formed of a movable part supported by a pair of springs, including a mirror surface and a coil pattern, and an optical writer using the optical scanner such as laser printers. The movable part is located in a bias magnetic field with a permanent magnet and the coil pattern is energized to sinusoidally oscillate the movable part back and forth.
An optical scanner including a oscillator having two elastic deformation modes, i.e., a deflection deformation mode and a twist deformation mode and a mirror surface is known as well. The oscillator is oscillated with a resonance frequency of each mode and the mirror surface of the oscillator reflects a beam to scan. For example, Japanese Patent No. 2981600 discloses an optical scanner including a controller controlling a spring constant trimmer so that the resonance frequency of the oscillator can automatically be adjusted to a predetermined frequency.
As mentioned above, various optical scanners in which a microscopic mirror formed by micromachining technology is torsionally oscillated to scan have been introduced recently. For a conventional mechanical element needing high-speed operation, inertia thereof has been a large obstructive factor to the drive speed. Particularly, a mechanical element rotationally oscillating in a predetermined angle needs reducing inertial moment. Then, it is necessary not to decrease rigidity of the mechanical element. For this purpose, the mechanical element has a hollow structure and a reinforced member is fixed thereon.
A microscopic mirror used in optical scanners, which needs high functionality and compactness requires being high-speed drivable and having high rigidity. When the rigidity is insufficient, the mirror largely deflects by an inertial force generated with oscillation thereof. Such a dynamic deflection noticeably deteriorates chemical properties of reflected light from the mirror. The mirror typically has thicker thickness to have higher rigidity to decrease dynamic deflection.
However, an actuator used in the optical scanner has a very small operational (electrostatic, electromagnetic and piezoelectric) force. When the mirror has thicker thickness to decrease the dynamic deflection, the inertia becomes large and a deflection angle largely lowers with a small drive force of the actuator. Therefore, the inertial moment of the mirror needs to be small to enlarge the deflection angle.
Japanese Patent No. 3740444 discloses an optical deflector in which a movable plate has a thickness becoming smaller outward in stages to reduce the inertial moment.
As mentioned above, the microscopic mirror used in optical scanners, which needs high functionality and compactness requires being high-speed drivable and further having high rigidity to prevent itself from largely deflecting by an inertial force generated with oscillation of the mirror. The optical deflector disclosed in Japanese Patent No. 3740444 reduces the inertial moment, having a movable plate with a concave portion located far from its torsion axis so that the movable plate is formed of less silicone in weight. A concave portion is not formed near the torsion axis of the movable plate so as to increase its solid section. When torsionally oscillated, the larger bending moment is thought to be loaded on the movable plate and the solid section near the torsion axis is increased to keep rigidity of the movable plate.
However, it is required to further decrease weight of the oscillation plate to further reduce the inertial moment and dynamic deflection thereof when oscillating while keeping the rigidity thereof.
Because of these reasons, a need exists for reducing inertial moment and dynamic deflection of a mirror substrate included in an optical scanner.